small letter vs lowercase

small letter and lowercase both are nouns.

Word	Noun	Adjective	Verb	Adverb
small letter	Yes	No	No	No
lowercase	Yes	Yes	No	No

As nouns, small letter and lowercase are synonyms defined as:

small letter and lowercase: the characters that were once kept in bottom half of a compositor's type case

Other synonyms of small letter include lower-case letter, minuscule.

small letter (noun)	lowercase (noun)
the characters that were once kept in bottom half of a compositor's type case	the characters that were once kept in bottom half of a compositor's type case

small letter (adjective)	lowercase (adjective)
	relating to small (not capitalized) letters that were kept in the lower half of a compositor's type case